The first invention concerns a growth method for a bulk II-VI type semiconductor material. It is especially applied for obtaining large-sized CdTe or CdZnTe plates. Such plates may be used in the manufacture of elementary or matrix detectors for X and xcex3 imaging.
At the present, the demand for X and xcex3 detectors is becoming ever greater whether it be under the form of elementary detectors or under the form of large surface single dimensional or two-dimensional image recorders. These devices generally use CdTe material (or rather CdZnTe) obtained either using the Travelling Heater Method (THM) with a chlorine doping, or the High Pressure Bridgman method (HPB) with the appropriate doping. In any case, these two methods present a certain number of drawbacks. The uniformity of the electrical properties is not controlled. The reproducibility of a pulling onto the other is not properly ensured. The THM method uses a closed tube and provides ingots limited to a diameter of 2 inches (5.08 cm) and a resistivity limited to 109 ∩.cm. The HPB methods uses an autoclave and provides ingots 4 inches (10.16 cm) in diameter but with a highly random grain size. It requires a high pressure (around 100 atmospheres), which is highly restrictive. Finally, the ingots have a resistivity close to 1010 xcexa9.cm, but not uniform along the ingot.
Cadmium telluride may also be obtained by solvent evaporation. The following articles refer to the growth of CdTe crystals following this principle:
xe2x80x9cPreparation of CdTe Crystals from Near Stoichiometric and Cd Rich Melt Compositions under Constant Cd Pressurexe2x80x9d by M. R. LORENZ, appearing in the Journal of Applied Physics, Vol. 33, No. 11, November 1962, pages 3304-3306;
xe2x80x9cGrowth of Cadmium Telluride by Solvent Evaporationxe2x80x9d by B. LUNN, appearing in the Applied Physics Review, Vol. 12, February 1977, pages 151-154;
xe2x80x9cCrystal Growth and Characterisation of Cadmium Telluride: a Modified Solvent Evaporation Techniquexe2x80x9d by J. B. MULLIN et al, appearing In Journal of Crystal growth, 59 (1982), pages 135-142;
xe2x80x9cGrowth of CdTe by Solvent Evaporationxe2x80x9d by A. W. VERE et al, appearing in Journal of Crystal Growth 72 (1985), pages 97-101.
These articles report on the use of cadmium as a solvent and a crystal growth performed in a sealed quartz tube in which a high-vacuum (less than 10xe2x88x926 atmospheres) is achieved beforehand. The need to control the cadmium pressure forces the introduction into the tube (or ampoule), before it is sealed, of a source of cadmium. Due to this fact, LORENZ was forced to develop an experimental system with three temperature zones which cannot be independent and with a displacement of the quartz ampoule. The authors of the other three articles chose a system with two temperature zones which cannot be independent and without any displacement of the quartz ampoule. One zone is centred over the crucible where the growth takes place, the other one over the cadmium source.
Until now, bulk CdTe growth has been done using the so-called closed tube method. The so-called open tube method has been reserved till now for the growth of thin layers of CdTe on a substrate. Bulk crystal growth and the growth of a thin layer on a substrate belong to two different fields of metallurgy for II-VI type semiconductors.
The problem that is posed is therefore how to obtain crystals of II-VI type semiconductor material (especially those made of CdTe and CdZnTe), which is bulky, with a large size and high resistivity, according to a growth method that is not too restrictive.
The invention allows a solution to be provided for this problem by proposing a growth method allowing the production of plates of bulk II-VI type semiconductor material, with a diameter greater than 10 cm and a very high level of resistivity (greater than 109 xcexa9.cm).
Therefore the purpose of the invention is a growth method for a bulk II-VI type semiconductor material including at least one first component and one second component, with the procedure including the following steps:
supply in a crucible of a charge including said components, with the proportions of the components in the charge being such that the first component may be used as a solvent,
placing of the crucible in a so-called xe2x80x9copen tubexe2x80x9d reactor,
raising of the reactor temperature to obtain a temperature profile in the reactor ensuring the melting of the charge in the crucible and with the evaporation of the first component beginning, with the pressure inside the reactor being adjusted by the circulation of a gas so that the partial pressures of the components will be lower than the atmospheric pressure, with the partial pressure of the first component being greater than the partial pressure of the second component, the temperature profile being such that the melted charge is kept at the growth temperature equal to or slightly higher than its balance temperature and lower than the melting temperature of the desired material, with the temperature profile also being such that the crucible will show a cold point where the germination and the growth of the material takes place, with the reactor also having a cold point thereby allowing the condensation of the solvent.
According to a preferred embodiment, the raising of the temperature is carried out in two successive periods:
a first period during which the temperature profile is such that the charge in the crucible begins to melt and the first component begins to evaporate, with one zone in the reactor located between the crucible and the cold point allowing the condensation of the solvent to be raised to a temperature higher than the temperature in the crucible so as to form a heat barrier;
a second period during which the charge in the crucible is brought up to said growth temperature, with said zone in the reactor located between the crucible and the cold point in the reactor being kept at a temperature lower than the growth temperature in order to allow the condensation of the solvent.
Advantageously, the temperature profile is ensured by a furnace that has two heating zones. Preferentially, the temperature profile is ensure in an upright furnace that has an upper heating zone, corresponding to the position of the crucible, and a lower heating zone located between the position of the crucible and the cold point in the reactor.
The growth temperature, in particular, may be a constant temperature.
The pressure on the inside of the reactor may be adjusted according to the desired evaporation speed for the solvent. This allows the growth speed for the semiconductor material to be controlled.
The pressure on the inside of the furnace may be adjusted by the circulation of argon.
The charge supplied in the crucible may also include at least one doping impurity, the vapour pressure of which is less than the vapour pressure of the components.
The charge supplied in the crucible may also include at least one doping impurity selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, Al, In, Ag, Au, Tl, Si, Sn, Pb, Bi, Li, Na, K, Be, Mg and Ca.
At the time of supplying said charge in the crucible, a seed may be introduced in the bottom of the crucible to favour single crystal growth.
The invention is especially applied to obtaining bulk CdTe. In this case, the first component is tellurium and the second component is made of cadmium.
The invention is especially applied again to obtaining bulk CdZnTe. In this case, the semiconductor material also includes a third component that is zinc.